Hardware Required

Circuit

Schematic

The SCP1000 barometric pressure sensor can read both air presure and temperature and report them via the SPI connection. For details of the control registers, see the SCP1000 data sheet.

In the code below, the Arduino starts out by setting the SCP1000's configuration registers in the setup(). In the main loop,it sets the sensor to read in high resolution mode, meaning that it will return a 19-bit value for the pressure reading, and 16 bits for the temperature. The actual reading in degrees Celsius is the 16-bit result divided by 20.

Then it reads the temperature's two bytes. Once it's got the temperature, it reads the pressure in two parts. First it reads the highest three bits, then the lower 16 bits. It combines these two into one single long integer by bit shifting the high bits then using a bitwise OR to combine them with the lower 16 bits. The actual humidity in Pascals is the 19-bit result divide by 4.

/*
SCP1000 Barometric Pressure Sensor Display

Shows the output of a Barometric Pressure Sensor on a
Uses the SPI library. For details on the sensor, see:
http://www.sparkfun.com/commerce/product_info.php?products_id=8161
http://www.vti.fi/en/support/obsolete_products/pressure_sensors/

//Read from or write to register from the SCP1000:unsignedint readRegister(byte thisRegister,int bytesToRead ){byte inByte =0;// incoming byte from the SPIunsignedint result =0;// result to returnSerial.print(thisRegister,BIN);Serial.print("\t");// SCP1000 expects the register name in the upper 6 bits// of the byte. So shift the bits left by two bits:
thisRegister = thisRegister <<2;// now combine the address and the command into one bytebyte dataToSend = thisRegister & READ;Serial.println(thisRegister,BIN);// take the chip select low to select the device:digitalWrite(chipSelectPin,LOW);// send the device the register you want to read:
SPI.transfer(dataToSend);// send a value of 0 to read the first byte returned:
result = SPI.transfer(0x00);// decrement the number of bytes left to read:
bytesToRead--;// if you still have another byte to read:if(bytesToRead >0){// shift the first byte left, then get the second byte:
result = result <<8;
inByte = SPI.transfer(0x00);// combine the byte you just got with the previous one:
result = result | inByte;// decrement the number of bytes left to read:
bytesToRead--;}// take the chip select high to de-select:digitalWrite(chipSelectPin,HIGH);// return the result:return(result);}

//Sends a write command to SCP1000

void writeRegister(byte thisRegister,byte thisValue){

// SCP1000 expects the register address in the upper 6 bits// of the byte. So shift the bits left by two bits:
thisRegister = thisRegister <<2;// now combine the register address and the command into one byte:byte dataToSend = thisRegister | WRITE;

// take the chip select low to select the device:digitalWrite(chipSelectPin,LOW);

SPI.transfer(dataToSend);//Send register location
SPI.transfer(thisValue);//Send value to record into register

// take the chip select high to de-select:digitalWrite(chipSelectPin,HIGH);}